Just beginning modding and here's a few programs I've found useful...

Aorta - http://sourceforge.net/project/showfiles.php?group_id=179330 - free
Converts .tga files to .dds, useful for changing some skins

Graphic Converter - http://www.lemkesoft.com/ - shareware $30
Can open and edit .dds files. Also exports to other formats. It seems to me that you need to convert .dds to .tga files for Aorta to convert them back to .dds. *shrug*

Textwrangler - http://www.barebones.com/ - free
Gremlin zapper

Smultron - http://smultron.sourceforge.net/ - free
XML editor. Much easier on the eyes than Textedit.

GimpShop - www.plasticbugs.com - free
GIMP compiled like Photoshop. Free alternative to Photoshop. Requires X11.

Python - http://www.python.org/download/mac/ - free
Latest version of Python. Seems like peecee .py files will work.

Blender - http://www.blender.org/ - free
Free 3D modeler. Haven't worked much with it. I think there's a plugin for exporting .nif files. *shrugs again*

Hope these help.